The present invention relates to an aqueous sizing composition for glass fibers and the sized glass fiber strand useful in reinforcing thermosetting polymers and the reinforced thermosetting polymeric material, where the sized strand in chopped form has good strength and good flow properties and leads to reinforced thermosetting materials with good strength.
Reinforced thermosetting polymeric composites can be reinforced with several forms of glass fibers including roving, continuous strand, chopped fibers or strands, mat, woven fabrics and the like. The glass fibers used as reinforcement for thermosetting polymeric materials are formed by being drawn at a high speed from molten cones of glass from tips of small orifices in a platinum-containing device known as a bushing. In order to protect the glass fibers from interfilament abrasion during formation and further processing and to make them compatible with polymeric materials like the thermosetting resins, a sizing composition is applied to the glass fibers during their formation. The sized glass fibers are then gathered into strands, and chopped during the forming process, a process known as the wet chop process, or wound into a package of continuous strand or strands known as a forming package. The wet chopped glass fiber strands or forming packages are then dried. The dry continuous strand of the forming package can then be chopped or combined with other dry continuous strand to form a roving, which may be subsequently chopped. The chopping of the dry continuous strand, strands or roving is a process known as dry chopping.
The sized glass fiber strands in these various forms are useful in producing glass fiber reinforced thermosetting polymeric composites. These composites are produced from thermosetting molding compounds such as bulk molding compounds and sheet molding compounds. The bulk molding compound is a combination of resin, short glass fibers usually having a length from about 1/8 to 1/2 inch, fillers, pigment, catalyst, thickness and other additives processed in bulk form in a high shear mixer or extruded into logs or ropes. The bulk molding compound is also available as a premix, which usually contains less specialized additives. The bulk molding compound typically has a glass content of around 10 to 25 weight percent. The sheet molding compound is a combination of resin, filler, chopped strand reinforcement, release agent, and catalyst processed in sheet form. Chemical thickeners such as alkaline earth oxides and hydroxides are used in sheet molding compound to increase the viscosity of the tack-free state. For use in sheet molding compound, the glass fiber strand roving is usually chopped to lengths of about 0.5 to 1 inch. The glass content of sheet molding compound typically ranges from about 25 to about 45 weight percent.
The processes used for producing these molding compounds place many demands for specific properties of the sized glass fiber strand. For example, the bulk molding compound is prepared by blending the short length chopped glass fiber strands into a putty prepared from a polyester resin or other thermosetting resin, fillers and other additives. This mixture is then mixed in the high shear mixer or extruded in such a manner to adequately disperse the glass fiber strands in the resin mixture. The bulk molding compound is then available in the bulk form for compression or injection molding or for extruding into logs or ropes of preforms cut to weight for compression or transfer molding. The bulk molding compound is typically molded at pressures from 300 to 1500 psi (20 to 102 atmospheres) and temperatures from 225.degree.-300.degree. F. (107.degree.-149.degree. C.). Cycle times range from 0.5 to 5 minutes depending on catalyst, temperature and thickeners of the composite part. The mechanical properties of the bulk molding compound are considerably less than the mechanical properties of sheet molding compound and products of other molding processes. The reasons for this are threefold. First, there is a low glass content in the bulk molding compound, second, the glass fibers have a short fiber length and third, the glass fiber strands tend to break up during the torturous mixing in the high shear mixer such as a sigma blade-type mixer.
Because of the rigors of processing such molding compounds, the formation of acceptable glass fiber reinforced molding composites demands the use of a proper sizing composition on the glass fibers. These sizing compositions must perform the dual function of protecting the glass fibers from abrasion during formation and further processing, while making the glass fibers compatible with the polymeric materials they are to reinforce. In the case of bulk molding compounds, the sizing compositions must also retard the break up of the glass fiber strands during the high shear mixing, which is necessary to disperse the glass fiber strands in a mixture containing the thermosetting or thermosettable resin. Typically, the sizing compositions for glass fibers are aqueous compositions having lubricants, film formers, coupling agents, wetting agents, emulsifiers and the like.
It would be beneficial to have a sized glass fiber strand that when they are in a chopped form have good flowability. Such a characteristic would enable the chopped glass fiber strands to easily move relative to one another when the chopped glass fiber strands are added from a source to a molding compound. Such a characteristic for glass fibers used in producing bulk molding compound would enable the chopped glass fiber strand to become dispersed more easily throughout the thermosetting resin mixture, and, thereby, reduce the amount of severe shear the chopped glass fiber strands are subjected to in preparing bulk molding compounds.
In addition, the sizing composition influences the behavior of the sized strands in producing chopped glass fiber strands as chopped strands or chopped roving. The sizing composition affects the choppability of the glass fiber strand by making the strand well integrated so that the strand is not easily filamentized when chopped. Also, the strand integrity given to the strand by the sizing composition should be retained after chopping, since it may affect the flowability of the chopped fibers, a characteristic that impacts on the removal of air during molding. The size present on the sized glass fiber strand to be chopped should provide this integrity between the strands while concomitantly preventing excessive generation of fly and fuzz when the strand is chopped.
It is an object of the present invention to provide an aqueous sizing composition for glass fibers and to provide sized glass fiber strand produced therewith that has good choppability to yield chopped sized glass fiber strand that has good strength and flow properties for use in producing thermosetting molding compounds for injection, compression, extrusion and transfer molding so that the molded reinforced thermosetting composite has good physical properties.